Drying of sticky particles is well known in the production of cereal. In such a production the cereal dough may first be extruded, rolled out to the appropriate thickness and cut into pieces or particles. The particles are then dried before they are puffed at a high temperature and subsequently coated with flavoring or sweeteners. At the drying step described here the particles of un-baked or wet dough will be sticky.
Drying sticky particles such as the pieces of cereal dough mentioned above or other related products e.g. dough for chip products, pasta dough etc. is difficult because the particles tend to stick together and form agglomerates, i.e. bigger lumps of dough. Once an agglomerate is formed, a non-uniform drying of the dough will take place. Furthermore, the shape of particles or chips will be ruined if dough particles are allowed to stick together during the drying process. This is unsatisfactory in the production of quality products.
Difficulties are also experienced when dough particles or chips having a high moisture content, e.g. 15 to 20% H.sub.2 O, are dried at a high temperature, e.g. about 180.degree. C., and contact is allowed between the particles while the drying takes place. In order to avoid this and to prevent cracks in the surface of the particles, they must be dried at lower temperatures. Conventionally, the drying operation of such particles has therefore been carried out in a number of steps.
Different processes are known for carrying out the drying of sticky particles. The first drying step may for example be drying in a tumbler dryer where the temperature is kept relatively low, e.g. about 100.degree. C. The particles or chips are only partly dried in this first drying step. Additional drying steps at higher temperatures may then be performed in tumbler dryers, fluid beds, etc.
The tumble drying required for drying of cereal dough in a production line usually is of large dimensions. For example, the dryer drum can be from 5 to 6 m long and about 2.5 m in diameter.
In the fluid bed only a thin layer, for example about 5 cm, of particles can be advanced and dried at a time. If the layer is too thick the sticky particles will tend to form agglomerates and will not be uniformly dried. The agglomeration of the particles hence limits the capacity of the fluid bed.
The above described multi-step drying processes, consequently are disadvantageous as they are both time and energy consuming.
An attempt at having a number of different drying processes in one apparatus is disclosed in U.S. Pat. No. 4,910,880. The apparatus described therein employs drying gases flowing up and down and which pass through a foraminous conveyor advancing the product to be dried. However, this patent does not describe any attempt to prevent agglomeration of sticky particles.